The transmission of information by light passing through space has been implemented for example by IrDA (Infrared Data Association). In particular, the divergent type of transmission method, in which light is transmitted in a divergent beam, does not require optical axis collinearity, and allows the provision of transmitter and receiver to be simple.
With such a divergent type of transmitter, for information to be transmitted optically through space, a large light output is required even for close range operation (not exceeding 1 m), and if the transmission distance is 10 m or so, a light output of at least 500 mW is required.
As the information transfer rate increases, this requires broad band transmission and a high frequency, and to detect light modulated at a high frequency requires the receiving area of the receiving optics to be small, and the electrical capacitance of the element to be small. For example, when transmitting information in the 1-GHz waveband, if a photodiode is used as the photodetector, light-receiving area must be not more than 0.1 mm.sup.2. As a result, for broad band optical transmission, because the light-receiving area is small, a larger light output is required.
On the other hand, in the beam type of transmission method in which the light is transmitted along a straight line, the light output can be less than for the divergent type, but the alignment of the optical axis is difficult. In particular, if the positions of the transmitter and receiver move, a mechanism for following the light beam is required. As the mechanism for following the light beam, for example, Japanese Patent Application Laid-Open No. 8-181654, discloses that an optical axis is adjusted by detecting a target to which the light beam is directed on the basis of image information obtained by using a lens and a charge-coupled device (CCD).
However, not only does such a CCD have a high power consumption, but it is not practical for the transmitter to be combined with a complex element such as a CCD, and moreover, the analysis of the image information, the detection of the target, and control of the direction of the light beam require a complicated algorithm.
The present invention solves the above-mentioned problems, and provides a beam type of spatial optical transmission device and method of spatial optical transmission such that the light beam can be tracked with simple control.